You will also find this page by searching for the keywords "revit devtv". The DevTV Introduction to Revit Programming is a self-paced video tutorial demonstrating how to get started developing with Revit. This page also points to the public Revit API discussion group and the possibility to join ADN, as well as providing links for downloading the SDK or Software Developers Kit, which is also included in every Revit product installation.
Happily, from Revit 2009 onwards, absolutely everything you need to get started with the Revit API is included with the basic Revit product. If you want to write a professional application, you will additionally need a .NET development environment. However, for exploring the API and creating macros, the built-in VSTA or Visual Studio Tools for Applications editor can be used.
The DevTV recording is based on Revit 2008. The principles remain the same in Revit 2009, except that element access has been tremendously speeded up by the implementation of filtered access.
We made a recording of the Revit 2009 API webcast that we held in May 2008. It is available from the public Developer Center course schedule (look for Revit API and the Download link) as well as from the ADN web page. It covers more or less the same material as the DevTV recording, and additionally discusses the filtering features added in the 2009 API. The recording material also includes the presentation itself and the source code of the individual sample commands or labs.